In my prior U.S. Pat. No. 3,333,967, now reissue U.S. Pat. No. Re. 28,995, a method is disclosed for preserving mature but less than fully ripe fruit which produce ethylene and are ripened thereby, using hypobaric conditions of about 100 to 400 mm HgA pressure in a flowing stream of humidified, nearly water-saturated air. This method gave useful results on a laboratory scale and under favorable conditions on a somewhat larger scale, but not when the size was increased and the pressure reduced below 100 mm HgA to improve the storage performance with certain mature but less than fully ripe fruit and to extend the utility of the method to other types of metabolically active matter, including animal matter. The evaporative cooling effect when air is contacted with a body of water which is relatively smaller in relation to the whole storage space than was the body of water in relation to the size of a conventional laboratory vacuum vessel decreases the temperature of the humidifying water. In my prior U.S. Pat. Nos. 3,810,508 and 3,913,661 with W. Hentschel, I disclose how to use and profit by this cooling to lessen the work of, or eliminate other ways and means of cooling the chamber, but refrigerating by evaporating water runs counter to the objective of creating and maintaining high humidity. As the water cools, its vapor pressure is lowered and it tends to add progressively less moisture to the incoming air so that the relative humidity in the chamber is reduced and the product dries and spoils prematurely.
In my prior U.S. Pat. Nos. 3,958,028 and 4,061,483 I disclose how to overcome the evaporative cooling effect and provide a constant high relative humidity in the storage chamber. Incoming expanded atmospheric air is preconditioned to the pressure and temperature inside the vacuum chamber, and then it is contacted with a body of heated water to saturate the chamber atmosphere. I also disclose that a relatively broad spectrum of correlated hypobaric pressures and low temperatures at a high relative humidity is operational in preserving metabolically active matter at pressures ranging form 4 to 400 mm HgA.
The humidification step of U.S. Pat. Nos. 3,958,028 and 4,061,483 has prevented the development of economically useful commercial hypobaric intermodal transportation containers which embody the equipment and method of these older patents. It is impractical to displace valuable cargo with the weight and volume of water which is needed to continuously saturate the incoming dry expanded atmospheric air changes during a prolonged oceanic trip, so instead a smaller amount of stored water has to be recycled by reclaiming the moisture which condenses when the saturated chamber atmosphere is exhausted by and compressed in a vacuum pump. Though otherwise preferable, oil-seal vacuum pummps cannot be used for this purpose because their exhaust is contaminated with oil, making water reclamation unfeasible. Instead, contemporary 40.times.8.times.8 foot prototype hypobaric intermodal transportation containers are equipped with a water-sealed vacuum pump, which is injected with enough refrigerated seal water to promote and sustain a nearly isothermal compression, and supercharged with a Roots vacuum pump to improve efficiency at low inlet pressures. The refrigeration equipment needed to cool the seal water uses nearly 5 KW of electric power, the supercharged vacuum system about 3 KW more than an oil seal vacuum pump having comparable effective displacement, and the heating element in the humidifier up to 2 additional regulated KW. Not only does the humidification step more than double the size of the refrigeration and motor generating systems which are needed, but in addition it creates a requirement for automatic boiler filling and flushing systems, automatic drains and heating tapes to prevent equipment freeze-up during periods of operation and inoperation at ambient temperatures lower than 0.degree. C., and complex safeguards and controls to integrate the systems. In this manner the humidification step increases the equipment and operational costs, reduces reliability and maintainability, and increases the transportation cost per pound of metabolically active matter.